The present invention generally relates to processing a semiconductor substrate. In particular, the present invention relates to a system and method for examining a wafer printed by a reticle in order to inspect for defects on the reticle.
Achieving the objectives of miniaturization and higher packing densities continue to drive the semiconductor manufacturing industry toward improving semiconductor processing in every aspect of the fabrication process. Several factors and variables are involved in the fabrication process. For example, at least one and typically more than one photolithography process may be employed during the fabrication of a semiconductor device. Each factor and variable implemented during fabrication must be considered and improved in order to achieve the higher packing densities and smaller, more precisely formed semiconductor structures.
In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist, and an exposing source (such as optical light, X-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the photoresist mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through the photoresist mask causes a chemical transformation in the exposed areas of the coating thereby making the image area either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer. The resulting pattern image in the coating, or layer, may be at least one portion of a semiconductor device that contributes to the overall structure and function of the device.
Because the photoresist is used to form features on the semiconductor devices, the integrity of the photoresist must be maintained throughout the lithography process. That is, any flaw or structural defect which is present on a patterned photoresist may be transferred to underlying layers during a subsequent etch process wherein the photoresist is employed.
Such flaws and/or structural deformities may be caused by a defect-ridden reticle which is used to pattern the photoresist. Reticle defects may be generated by the fabrication process utilized to produce the mask or reticle as well as during subsequent handling and processing. Reticle defects can often lead to repeating defects on a wafer. A repeating defect is a defect that appears in multiple wafers or layers at about the same location. The repeating defect can be an indicator that a reticle has a defect, as opposed to the defect being caused by a one-time contamination problem or by wafer-to-wafer variations.
With the increasing use of advanced reticle enhancement techniques, the effect of defects, even marginal defects, can be magnified when transferred to a wafer. If the photomask or reticle contains defects, even submicron in range, such defects can be transferred to a wafer during exposure. Defects on reticles may cause inaccurate patterns to form on the wafer. In addition, the electrically active regions may not perform as desired, leading to an overall degradation of chip performance.
For example, closed area defects such as malformed or undeveloped edges, corners and lines as well as undesired depressions, dimples, protrusions and pinholes in the layers may adversely affect the performance of the semiconductor. Adverse effects may include increased resistance, decreased capacitance, ineffective insulation between layers and features, and poor conductivity and interconnections between layers and features.
Conventional inspection tools have been developed to detect defects on the reticle. More recently employed inspection systems utilize defect simulation methods to detect repeating defects in reticles. The defect simulation inspection method involves employing an aerial image microscope system to simulate printability on a wafer. This method compares an optical image with a simulated image derived from the original design data and generated with an aerial image microscope system and predicts how features and defects on reticles will print on full flow production wafers.
However, defect simulation comparisons are not as accurate as die-to-die inspections in pinpointing the precise shape and location of the reticle defect. Furthermore, simulated images involve a considerable amount of image processing power. Such automatic image processing instruments often have difficulty detecting particles on substrates that have defect simulation inspection method requires costly instrumentation capable of comparing an actual photomask geometry against an ideal photomask geometry.
Die-to-die methods involve patterning and developing a resist wafer and then performing a wafer inspection by SEM or by optical methods from die to die to check for any repeating defects. Although typical die-to-die methods are preferred for their accuracy, it is a time consuming and rather slow process. Thus, fabrication times undesirably increase, thereby unduly inflating overall production costs and processing inefficiencies.
Therefore, there continues to be an unmet need for a rapid and more accurate process to assess marginal defects on a reticle in order to increase fidelity in pattern transfer and to improve device performance.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention provides a system and method for examining a reticle for defect printability. More specifically, the present invention involves a system and method for inspecting a reticle for marginal defects by comparing latent images printed on a resist wafer. Marginal defects on the reticle may or may not be destructive to the final device structure, however, in order for this determination to be made, it is desirable to locate and qualitatively analyze such defects in a relatively rapid manner. This may be accomplished in part by using the reticle to print latent images onto the resist wafer and then examining the latent images for defects. The latent images are inspected before the resist is developed. Because the resist wafer is not developed, processing time is reduced and the status of the reticle can be more readily determined.
According to one aspect of the present invention, the reticle may have one die pattern. The one die pattern can be printed at least three times on the photoresist layer at three adjacent locations to form three latent images on the photoresist layer. The three latent images may be compared to one another to establish whether the images exhibit measurable differences from one another. Comparison of the three latent images may be optimized by varying the focal height during the printing of each latent image and/or by varying the exposure conditions for each latent image. Because photoresist materials differ in structural and physical properties, bleachable dye may also be employed in order to enhance the contrast between the printed and non-printed areas of the photoresist layer.
According to another aspect of the present invention, the reticle may have more than one die pattern. The die patterns may be similar or different depending on the desired end product device to be made. Inspection of the reticle for defects may be performed in a similar manner as described above.
The comparison between the three latent images assists in pinpointing and verifying the location of a defect on the reticle. In addition, the comparison facilitates determining the severity of the defect(s) on the reticle. If it is determined that the reticle is defective, then it may be discarded or require full verification by developing the wafer for conventional defect inspection. On the contrary, a qualitative analysis of the defects may also reveal that such defects are minor or inconsequential and not likely to affect the structure or operation of the device. In summation, the quality of the reticle may be ascertained without developing the resist wafer, thereby reducing overall processing time and production costs.